In its widest acceptance, a liquid crystal cell is essentially made up of a transparent front substrate and a back substrate that may also be transparent, or not transparent, these two substrates being joined and maintained at a constant distance from each other by a sealing frame, which defines a sealed volume for retaining the liquid crystal. Further, the front and back substrates each include, on their faces opposite each other, at least one electrode, these electrodes being intended to be connected to an electronic control circuit which, by the selective application of appropriate voltages to determined electrodes, enables the optical properties of the liquid crystals to be altered at the point of intersection of the electrodes concerned.
One constant problem in the field of manufacturing liquid crystal cells of the type described hereinbefore is raised by the connection means for setting up the electrical connection between the electrodes of the cell and the control circuit. One simple technique for accomplishing the electrical connection between the electrodes and the control circuit consists in staggering the substrates in relation to each other, so that one can access a flat part of the electrodes and thus easily form the connection on the surface thereby freed. However, such an arrangement of the substrates makes large-scale manufacture of the cells difficult, particular when the latter are circular, and it requires additional time-costly operations. The resulting cells are also of larger dimensions and thus difficult to house inside portable electronic objects of small dimensions such as a wristwatch.
Another solution to the aforementioned problem has been proposed by the Applicant in the International Patent Application published under no. WO 99/41638. There is known, in fact, from this Application, an electro-optical cell such as a liquid crystal cell, or an electrochemical photovoltaic cell, these cells each being formed of a transparent front substrate, a back substrate that may also be transparent or not, a sealing frame joining the two substrates and defining a volume for retaining, in a sealed manner, a photo-electrically or electro-optically active medium, the substrates including on their faces opposite each other at least one electrode. These cells differ in that their electrodes are partially flush with the edge of the substrate on which they are deposited to define lateral electrical contact zones at the level of which right-angled shaped parts are added, which advantageously allow the electrical contact to be transferred to the back face of the cells.
Owing to these features, it is no longer necessary to stagger the two substrates of a cell to be able to establish the electrical connection between the electrodes of the cell and its control circuit, such that these substrates can be identical and arranged one overlapping the other. This thus results in great cell manufacturing simplicity, and thus to a considerable reduction in their cost price. Moreover, the space requirement of the cells is reduced, which facilitates the assembly thereof particularly in a wristwatch.
The solution described hereinbefore has, however, one drawback. In fact, the electrical contact parts, made of metal or a metal alloy that conducts electricity, form discrete elements that have to be bonded one after the other by means of a conductive adhesive onto the edge of the cell, at locations where the electrodes are flush with the edge of the substrate on which they are deposited. Such a manufacturing method, while relatively simple to implement in the case of an electrochemical photovoltaic cell which only comprises two electrodes, proves much more complicated to use in the case of liquid crystal cells, which, when they are of the matrix type, can comprise several hundred electrodes and counter-electrodes, which must each time be associated with a transfer electrical contact part. Of course, the manufacturing method could be automated, but the number and precision with which the electrical contact parts ought to be added onto the edge of the cells would considerably increase the manufacturing costs of such cells.
In order to overcome this problem, Patent documents JP 56 075624 and JP 56 168628 are known, which both disclose liquid crystal cells on the edge and back face of which conductive paths are deposited by printing.
Owing to these features, the display cell substrates can have substantially equal dimensions and do not need to be staggered with respect to each other to allow the electrical connections to be established between the electrodes and the control circuit. Thus, if, for the same external dimensions, the active surface of a liquid crystal cell can be increased, the display resolution can be increased, i.e. a larger number of line and column electrodes can be provided, without the pixel aperture, and thus the reflectivity of the display cell, being altered. Conversely, the increase in the display cell active surface, with a constant number of line and column electrodes, can be exploited to increase the width of the electrodes and thus the pixel aperture, which has a favourable effect on the reflectivity of the display.
The results described hereinbefore are attained owing to the fact that the means for connecting the electrodes of a display cell to a control circuit are not formed by contact parts which have to be individually bonded, but by conductive paths which allow the electrical contacts formed by the electrodes of a cell to be transferred from the edge of the cell to the back edge of the cell, then, from there, around the back edge and onto the back of the cell, these conductive paths being formed on the edge and the back of the cell by any appropriate technique for depositing and etching conductive layers in order to achieve the desired dimensions and resolution.
It might, however, be feared that the conductive paths, which are deposited in the form of thin layers, exhibit problems of reliability and electrical conduction continuity at the place where they match the often sharp back edge of the cell. In fact, the thermal stresses that appear during the cell manufacturing process can tear the conductive paths at the edge of the back substrate of the cell. Likewise, a thin layer deposited around the back edge of the cell has low mechanical strength and can easily be scratched and interrupted when the cell is handled or when one wishes to mount it in the portable object for which it is intended.
It is an object of the present invention to overcome the aforementioned problems in addition to others by providing a display device, particularly a liquid crystal cell, comprising means for connecting its electrodes to an electronic control circuit which are both simple to manufacture and reliable.